The present invention relates to braking systems for motor vehicles and relates more specifically to a brake booster.
Braking systems generally comprise a master cylinder intended to apply the pressure of the brake fluid to the brakes of a vehicle when a brake pedal is actuated. Inserted between the brake pedal and the master cylinder is a vacuum-type brake booster. Such a brake booster comprises, as is known, an enclosure, a diaphragm dividing the enclosure into a front chamber which is equipped with means of connection to a source of vacuum and into a rear chamber, a piston that can move axially in the enclosure and on which the diaphragm is mounted, a plunger mounted so that it can move axially in the piston under the control of a brake pedal between a forward unstable braking position and a retreated stable position of rest, and a valve collaborating with seats on the piston and on the plunger so as, on the one hand, to isolate the rear chamber from the ambient atmosphere and place it in communication with the front chamber when the plunger is in the position of rest and, on the other hand, to isolate the rear chamber from the front chamber and place the rear chamber in communication with the ambient atmosphere during braking.
Such a booster allows the braking force applied to the brake pedal to be multiplied by a multiplication factor which may, for example, be as high as 9. In this type of brake booster, the piston has a single seat of circular shape lying so that it is concentric with the seat borne by the plunger. The drawback of such an arrangement is that it disturbs the flow of air towards the front chamber.
The subject of the invention is therefore a brake booster of the aforementioned type, characterized in that each seat on the piston is formed on the edge of a passage allowing the communication between the chambers.
The shape of the seats therefore corresponds to the shape of the passages, and this makes it possible to make the flow of air between the chambers more uniform.
Furthermore, in one embodiment, it makes it possible to provide more direct passages for the air for filling the rear chamber with the air entering the booster from the outside.
The brake booster according to the invention may further comprise one or more of the following features, taken in isolation or in any technically feasible combination:
each passage has a transverse cross section in the shape of a truncated annulus;
the brake booster has two passages allowing the said chambers to communicate, these passages being arranged symmetrically with respect to the axis of the piston;
it has a single seat comprising two lateral parts each formed on the edge of one of the said passages and connected by a circular part coaxial with the plunger;
the brake booster comprises a first set of primary passages formed in a bore of the piston in which the plunger slides and which open towards the rear chamber and a second set of secondary passages designed to increase the flow of air between the rear and front chambers when the plunger returns to the position of rest;
the secondary passages open into the piston in a zone which lies in a plane which lies forward of a plane defined by the corresponding seats.
Another subject of the invention is a piston for a brake booster as defined hereinabove, comprising an internal bore in which a plunger is intended to be mounted so that it can slide between a forward unstable braking position and a retreated stable position of rest, at least one passage for establishing communication between a front chamber and a rear chamber which are delimited by a diaphragm fixed to the piston, at least one seat intended to collaborate with a valve mounted on the piston for closing the or each passage upon braking, the bore having second passages for establishing communication, upon braking, between the rear chamber and the ambient atmosphere, characterized in that the or each seat is formed on the edge of one of the said at least one passage that allows the said chambers to communicate.
Advantageously, each passage which allows the chambers to communicate has a transverse cross section in the shape of a truncated annulus.
As a preference, the piston comprises additional passages designed to increase the flow of air between the rear and front chambers when the plunger returns to the position of rest.